ENH: Add convergence tolerance option to VB
ENH: Add plot function to bayespy.plot module
BUG: Fix f in Gamma node
ENH: Implement general inner product

This inner product has the following features:
 * broadcasts
 * number of arrays is arbitrary
 * the number of axes over which the inner product is computed is
   arbitrary (ndim keyword argument)
MAINT: Return dims as a tuple instead of a list
ENH: Preliminary sketching of different Gaussian-Wishart nodes